The invention relates to an integrated circuit with a programmable locking circuit for the generation of an irreversibly changed switching signal after programming, comprising at least two EEPROM cells whose output signals produce the switching signal after an EXOR operation.
The data-file security in integrated circuits is protected by means of locking circuits. It is ensured that after programming no data can be read from the integrated circuit or can be manipulated therein.
From U.S. Pat. No. 5,175,840 an arrangement is known by means of which EEPROM cells can be tested and unauthorized access to the cells can be inhibited. The external data exchange is controlled via a security signal. In one state of the security signal external reading and writing of data is possible. The inverse state prevents the external data exchange. A first condition defining the state of the security signal is derived from the states of security EEPROM cells. When the states of these security EEPROM cells are all identical the security signal assumes a state, after an ENOR operation, which enables the external data exchange. Identical states of these cells occur only if previously all the cells have been erased or set. These states can be set externally. A second condition permanently sets the security signal in such a manner that an external data exchange is no longer possible. If one condition is fulfilled or if both conditions are fulfilled the security signal assumes a state in which no external data exchange is possible.
When the security signal allows an external data exchange the arrangement is in a test mode in which the data of the EEPROM cells can be read and rewritten from outside.
The security signal can be inverted as long as the erase and write modes for the security EEPROM cells can be set externally, as a result of which the security signal can also be inverted by non-authorized persons when the structure and programs are known. It is not until a specific external program voltage has been applied to a given terminal that the security signal becomes non-invertible and the EEPROM data in the integrated circuit can no longer be changed from outside. When this voltage is not applied the arrangement remains in a state in which the security signal can be reset.
It is the object of the invention to provide a locking circuit which generates a switching signal which is no longer invertible.
In an integrated circuit with a programmable locking circuit of the type defined in the opening paragraph this object is achieved in that the EEPROM cells are only programmable at the same time in such a manner that one cell is written in and another cell is erased.
For this purpose, the arrangement is based on an initial condition in which the EEPROM cells have not yet been programmed and have identical states. In this initial condition the switching signal can be set by means of a programming pulse so as to assume a state which is a locked state. In this respect, it is to be emphasized that it is merely required to program and the programming result is unambiguously defined by the cell arrangement. After programming, a change of the control gate voltage does neither allow restoring of the initial condition, in which the cells still have the same non-shifted and fabrication-related threshold voltage, nor inversion of the switching signal, by means of which locking is to be effected.